Liquefied atmospheric gases, including nitrogen, oxygen and argon, are finding increasing uses in industry. Such liquefied atmospheric gases provide cryogenic capabilities for various industrial processes, are more economical to transport in merchant supply and provide ready and economical sources of gaseous product from liquid storage facilities. For instance, liquid nitrogen is increasingly used to freeze food products, to cryogenically embrittle used materials for cleaning or recycle, and as a supply of gaseous nitrogen inerting medium for various industrial processes.
The cost of liquefied atmospheric gases is generally a factor in comparing the use of gaseous product and liquid product. It is apparent that additional energy or power to produce the necessary refrigeration to derive liquid products from air makes a liquid generating process more energy intensive than the typical gaseous product process. Therefore, to meet the increasing needs for liquid product in the area of atmospheric gases, it is desirable to have a process which is energy efficient in operation and economical from a capital cost factor. The prior art has frequently suffered from either or both of these aspects of producing liquefied atmospheric gases.
British Pat. No. 1,472,402 discloses a cryogenic air separation cycle wherein gaseous nitrogen is removed from a column, is liquefied in a separate system and the liquid is recovered in part as product and a part as reflux for the column.
In U.S. Pat. No. 3,605,422 an air separation process is described wherein liquid oxygen and liquid nitrogen are derived directly from a distillation column. All of the feed air to the process enters the high pressure stage of the distillation column. In addition, the liquid nitrogen product of the process is entirely derived from the high pressure stage of the column. The process also utilizes a gaseous nitrogen recycle refrigeration system including an external refrigeration package to provide sufficient cryogenic temperatures to produce adequate reflux in the distillation column and liquid product. This process is capital intensive in that it must require external refrigeration which additionally increases power requirements. The liquid nitrogen produced in the nitrogen recycle of this patent is returned entirely to the distillation column for further rectification when liquid oxygen is the predominant product.
In U.S. Pat. No. 4,152,130, a process is set forth for the production of liquid nitrogen and liquid oxygen from a cryogenic distillative separation of air using a two-stage column and an air recycle refrigeration package. Nitrogen recovered as the gas from the distillation column is not passed through a recycle mode, but is rewarmed and vented or used for clean-up duty. The liquid atmospheric gas products are derived directly from the column, specifically as liquid nitrogen from the reboilercondenser of the high pressure stage of the column and liquid oxygen from the sump of the low pressure stage of the column. Accordingly, the entire removal of liquid nitrogen product directly from the column affects the quantity of reflux available to operate the rectification of the various stages of the column and increases the total air process requirements for that flowscheme.
In U.S. Pat. No. 4,375,367, an improved process derived from the previously discussed patent is set forth which requires less capital expenditure due to the reduction in the utilization of compressor-expander apparatus for the production of refrigeration. The cycle utilizes a recycle of a portion of the feed air, as well as a rewarming and introduction into the low pressure stage of a high pressure nitrogen gas product from the high pressure stage of the distillation column. However, the entire nitrogen liquid product is derived from the reboiler-condenser of the high pressure stage of the column, and again, the production of such liquid product will be affected by the limitations that the requirement for reflux dictate on the process.
In U.S. Pat. No. 4,400,188, a cryogenic system for the recovery of gaseous nitrogen is disclosed. A gaseous nitrogen recycle is utilized in which nitrogen is condensed at the base of a column, rather than at an intermediate level, and the resulting liquid is entirely utilized as internal reflux.
In U.S. Pat. No. 4,464,188, a nitrogen recycle and an air stream are used to reboil a distillation column and the resulting liquid nitrogen and liquid air are both fed entirely to the column as reflux and feed respectively. Gaseous nitrogen is produced by the process.
Various of the problems of capital intensity, energy demands and recoveries in cryogenic distillative separations for the production of liquid atmospheric gases are overcome by the present invention by the design and methods set forth below.